The present invention relates to a coating apparatus. More particularly, the invention relates to an improved extruder which has a doctor edge in part of an area close to a front-most end and which continuously extrudes a coating solution so as to deposit a uniform thickness of coating material on the surface of a moving support as it is guided by the doctor edge.
The term "support" as used herein includes flexible sheets or webs such as plastic films, paper, polyolefin-coated paper, and metal sheets such as aluminum and copper. Such sheets or webs may be provided with a subbing layer. As well as being applicable to photographic film or paper, a support may be treated with a coating solution such as a magnetic paint or photographic coating solutions so as to provide a magnetic recording media.
While various coating systems are used today, the applicator of the extruder type with a doctor edge is very common and used in various fields (see, for instance, Japanese Unexamined Published Patent Application No. 138036/1975, Japanese Patent Publication No. 7306/1979, and Japanese Unexamined Published Patent Application No. 84771/1982).
The conventional extruder-type applicators have one common problem in that their operating speed ranges are very narrow. At coating speeds higher than about 100 to 150 m/min, all known previous methods have considerable difficulty in achieving consistent application of a coat having a wet thickness of 20 microns or less.
As a result of intensive studies made on this problem, the present inventors have found that the difficulty arises from the fact that if the running speed of the web is increased to about 100 to 150 m/min or higher, the amount of air that is entrapped by the extruder portion is appreciably increased. In order to provide a uniform thin coat in this range of coating speeds, it is important that the pressure of the liquid issuing from the slot be freely controllable. If only a low liquid pressure can be produced, air bubbles will enter the coat being formed, or the coating solution will flow backwardly, namely, upstream toward the extruding position. For either reason, the resulting coat is not uniform in thickness. If, on the other hand, only a high liquid pressure is possible, variations in thickness in the transverse direction can easily occur during application of the coating solution in small quantities. It has also been found that these phenomena are governed to a great extent by the shape of the edge portion, and that the known techniques and the system shown in the Japanese Unexamined Published Patent Application mentioned above can suffer both types of the problems mentioned above.
In order to eliminate these problems, the present inventors made various studies and previously proposed an applicator system of the extruder type in Japanese Unexamined Published Patent Application No. 104666/1983. A cross section of this system is shown schematically in FIG. 1, wherein the surface of a flexible support W running continuously along a back edge surface 1 and a doctor edge surface 2 is coated with a coating solution that is continuously extruded from an end of a slot 3. In this applicator system, the doctor edge surface 2 has a triangular cross section so as to enable application of the coating solution as it is slightly pressurized in a sink formed on the slot side of the doctor edge surface.
Since the coating solution in that sink is constantly pressurized during the coating process, the system prevents entrance of air at the rear edge surface and, hence, assures fast application of a uniformly thin film. However, if continuous application is carried out over long periods, streaks occur in the surface of the coat and produce a mottled appearance. This is responsible for the occurrence of adverse effects, and in the production of magnetic recording media, it impairs the S/N or C/N ratio of the final product.
The present inventors have found that such streaks are formed because of the presence of dirt or other foreign material transferred from the surface of the support (web) to the sink P and accumulated in that area as a result of extended operations. Dirt or other foreign material sticks easily to the surface of the support and cannot be completely removed by conventional cleaning techniques such as washing with water. Some such contaminant inevitably enters the coating solution being applied to the surface of the support. Since the doctor edge in the applicator shown in FIG. 1 has a triangular cross section, the dirt and other foreign material cannot be effectively ejected from the system by passing over the apex 4 of the triangle. Thus, the dirt or foreign material builds up in the sink P, causing streaks on the surface of the coating.
Therefore, the present inventors made various studies in order to develop an applicator system that retains the advantages of the system shown in FIG. 1 but which enables extended continuous operating periods without producing streaks on the surface of the coat. As a result, the inventors have found that by using a round (curved) doctor edge surface and designing a system wherein the liquid sink is held under pressure during operation, high-speed application of the coating solution can be realized without producing undesired streaks on the surface of the coat.